$6 SMITHSONIAN MISCELLANEOUS COLLECTIONS VOL. I39 



muscle field short and narrow, somewhat longitudinally rectangular in 

 outline ; diductor scars small ; adductor scars large and surrounded 

 anteriorly by the diductors. Vascula media strong, converging an- 

 teriorly on the tongue. 



Brachial valve interior with deep sockets bordered by overhanging 

 socket ridges ; crura attached to socket ridges by narrow outer hinge 

 plates ; crura falcifer, long crescentic in section, convex outward ; in- 

 ner hinge plates incipient, forming a slight thickening on the inside of 

 the crura near their proximal end ; median ridge or septum absent ; 

 adductor field small, rounded in outline with large anterior scars and 

 small posterior ones ; vascula media prominent, diverging widely at 

 the anterior end of the adductor field. 



Type species (by original designation). — Hemithyris sladeni Dall, 

 Trans. Linnaean Soc. London, ser. 2, Zool., vol 13, pt. 3, p. 440, pi. 

 26, figs. 7-12, 1910. 



Comparisons. — This genus is most like Basiliola in form and out- 

 line but differs in having anterior costation. Inside the pedicle valve 

 the dental plates are reduced to remnants or are wanting in the mod- 

 ern species. In the brachial valve the development of outer hinge 

 plates in Basiliola is usually greater than that in Rhytirhynchia but 

 otherwise the details of the valves are the same. Incipient inner hinge 

 plates appear in Rhytirhynchia. 



Rhytirhynchia in its anterior costation suggests Eohemithyris which 

 likewise has anterior costation in old adults. In the latter this seems 

 to be a rare feature but the two genera are not likely to be confused 

 because their lateral profiles are different, that of Rhytirhynchia hav- 

 ing an extremely deep brachial valve, whereas Eohemithyris has both 

 valves nearly equal. 



Geological range. — Pliocene to Recent. 



Distribution. — Rhytirhynchia occurs as a fossil in the Pliocene 

 of Okinawa and today lives in the Indian Ocean south of the Saya de 

 Malha banks. 



Assigned species. — Two species are now assigned to this genus, one 

 living and one fossil : 



Hemithyris sladeni Dall, Recent, Indian Ocean. 

 Rhytirhynchia hataiana Cooper, Pliocene, Okinawa. 



Discussion. — This genus is essentially a semicostate Basiliola. In 

 the one modern species the dental plates are remnantal but in R. hatai- 

 ana from the Pliocene of Okinawa the dental plates are moderately 

 developed. This is a small and delicate form in which internal 



